1. Field of the Invention
The present invention relates to a process for producing a composite sintered body, formed from materials that are the same or different, using a metal injection molding (hereinafter abbreviated to "MIM") method, or process.
2. Description of the Prior Art
In order to prepare a molded article with dimensional accuracy, it is a common practice to conduct a cutting operation. In some molded articles, however, the material is so hard or fragile that it is difficult to perform lathe machining.
Examples of such molded articles include magnet bases and yokes of motors formed using a soft magnetic material comprising an iron-silicon (Fe--Si) alloy and an iron-cobalt (Fe--Co) alloy. In these articles, since the material is hard and fragile, the production yield when cutting is low, which renders these materials unsuitable for practical use.
In this connection, there is a metal injection molding method (MIM method) that comprises mixing a metallic powder with an organic binder, subjecting the mixture to injection molding in a necessary shape, placing the resultant molded article in a furnace wherein the temperature is gradually raised to remove the organic binder through the decomposition of the binder, and raising the temperature of the molded article from which the binder has been removed, thereby sintering the molded article.
This method is characterized in that it is suitable for working a material of the type described above and is applicable to a molded article having a complicated shape, and affords a high yield.
In some metallic components prepared by the MIM method, from the viewpoint of properties and cost, it is preferable that, depending upon applications, the components do not comprise a single material.
For example, in a magnet base for a wire dot printer, it is not always necessary that a core portion, wherein a coil is wound and a current is applied so as to generate a magnetic flux, comprise a material identical to that constituting a yoke portion used for forming a magnetic flux path; from the viewpoint of properties and cost; it is preferable that the core portion comprise a material different from that constituting the yoke portion.
When a sintered body having a protrusion and a thick-wall portion is formed by the MIM method, deformation is liable to occur in the protrusion while cracking or blistering is liable to occur in the thick-walled portion. In this case, a sintered body can be produced with a good yield by forming a protrusion or a thick-walled portion by powder compression molding, placing the formed protrusion or thick-walled portion in a mold and then applying the MIM method.
The present invention relates to a process for producing the above-described composite sintered body.
The process for sintering an injection molded article comprises four steps, that is, the step of kneading raw materials, the step of injection molding, the step of removing the binder and the step of sintering.
Specifically, a metallic powder having a mean particle diameter of 10 .mu.m or less is well (i.e., thoroughly) kneaded with an organic binder such as paraffin wax, and a pressure of about 1 ton/cm.sup.2 is applied so as to conduct injection molding and provide a molded article.
Then, the molded article is heated to a temperature of about 400.degree. C. in a non-oxidizing atmosphere, such as argon (Ar) or nitrogen (N.sub.2), subjected to a treatment for removing the binder through the evaporation thereof, and then heated to a high temperature to conduct sintering.
In some components used in electronic equipment, it is preferable that the metallic components partly comprise different materials rather than a single material depending upon applications and shapes thereof.
In such applications, components have been prepared by preparing individual components by the MIM method and joining the components by means of screwing, soldering, diffusion joining or the like.
When such a method is applied, the shape, material, etc. of the joint are limited. In general, the bonding strength is weak, and the process steps are increased, which unfavorably cause problems such as increasing the production costs.
When a molded article having an uneven thickness comprising a thick-walled portion, a protrusion, a thin-walled portion, etc. is molded by injection molding and then sintered to give a sintered body, in the prior art, cracking or deformation is liable to occur, thereby lowering the production yield.
Accordingly, an object of the present invention is to improve the production yield.
Further, with respect to different materials, it is preferable to form a composite sintered body using the MIM method. In this case, cracking should not occur in the joint. Another object of the present invention is to provide a solution to this problem.